CDF-II ISL (Intermediate Silicon Layers)

The ISL is composed of five barrels in total, each barrel being composed of single layer of double-sided Si microstrip sensors.
An elevation view of the CDF-II detector is here.
The barrels are positioned at radius of 22.6/23.1 cm (central barrel), 19.7/20.2 cm (forward/backward inner barrels) and 28.6/29.0 cm (forward/backward outer barrels) such that there are coordinate measurements at two (one) positions for forward (central) tracks.
Each barrel is made up from ladders, each ladder consisting of 6 Si mircrostrip sensors which are ganged into two 3-sensor groups so that signals are read out from either end of the ladder.
The Si sensor is AC coupled and double sided having two planes with 112 micron pitch readout strips. The strips are running at a stereo angle of ~1.2 deg.
The main function of the ISL is to measure the particle momentum in the forward regions where the outer tracker, COT, can not fully cover and
to provide anchor hit points from which track segments in SVX-II/ISL detector are searched for.

Design of the ISL and the sensor

An isometric view of the ISL barrels is here ( GIF view ).
The microstrip sensors are processed on 4" or 6" wafers.
The dimensions of 4" prototype sensors from Hamamatsu ( GIF view) is determined to utilize the maximum available area. The dimensions of 4" SEIKO sensors are similar (the outer dimensions are identical).
Six of such sensors are aligned to construct a ladder.

The strips run along the beamline for the n-side and at a small stereo angle of 1.207deg for the p-side. This angle is determined such that the stereo strips are aligned on a line when the neighboring sensors are positioned at a gap space of 100 microns ( GIF view ).
The phi-acceptance of ISL barrels is plotted here ( GIF view). The plot shows how the acceptance is degraded with increasing the number of stereo strips which are not read out. 12 strips at the corner are short and not readout, though they are biased so that the field at the corner is not distorted. Wirebonding three sensors, 36 strips at the corner are not read out per half ladder. The plot shows the phi acceptance is not degraded at all - we have enough overlapping between the sensors.

The specifications of 4" prototype sensors are summarized in this list ( PS file). The pad position is shown here( GIF view ). A constraint concerning the pad position is the wire length be shorter than 3 mm. A scheme of staggering the pads is avoided to simplify the wirebonding procedure.

Details of the structure of the 4" ISL sensor

The design of the sensor is based on the various studies carried out in designing Hamamatsu SVX-II sensors. The implant strip width is 22 microns to match the strip pitch of 112 microns. The Al electrode width is set to 16 microns so that the Al electrodes are recessed by 3 microns from the implant strips, thus suppressing the micro-discharge.

The drawings above are for the SEIKO prototype. The main differences between the two manufactures are the materials at the coupling capacitance and the passivation. The differences are shown here ( GIF view , EPS file ).

Performance of the 4" ISL prototypes

Number of dead coupling capacitors

SEIKO judges dead coupling capacitance from the leak current (0.1 microA) when
100 V is applied across the DC and ADC pads. The capacitor can stand at
higher voltages but bad capacitors show leakage current at this voltage.
The number of dead capacitaors is histogrammed for the 9 prototype sensros.

Hamamatsu judges dead coupling capacitance from transient current induced when
a pulse of 100 V is applied across the dedicated strip located at ends of implant strips and AC pads at the other ends. This way short capacitors (large currents), open Al electrode (small currents) and contacting Al electrodes (current larger scaling with the number of contacting strips) can be detected.

Poly-Si resistance of 9 SEIKO prototype sensors

The resistance was measured for test structures located outside the sensor. There are five structures per side uniformly distributed corresponding to the resistors at the middle, sides, and in between.

leakage current from individual channel

The leakage current induced at individual channel is measured by probing
the individual DC pad while bias voltage of 120 V (Hamamatsu) or
100 V (SEIKO) is applied.
Measurement is done only for the p-side, since the device can be biased
from the same side only on p-side. The sample Hamamatsu-18 has only one
leaky channel, and the sum of the all individual currents is equal to the
total leakage current measured at 120V, 0.3 microA. The two samples by SEIKO
show some clusters of leaky channels.

Interstrip capacitance vs. bias voltage (frequency dependence)

The interstrip capacitance is measured in two configurations: C1 is the capacitance between two neighboring Al strips while the others are floating, C2 is the capacitance between one Al strip and one neighboring strips on each side while the others are floating.

The strips in the above measurements are the one at the middle of the sensor.

Performance related to Co-gamma irradiation

Co irradiation was performed on January 26, 1998 for two sensors each for Hamamatsu and SEIKO.

Total leakage current during and after exposure

The sensors were exposed at a fixed rate of 35 krad/h for a total of 5 hrs.
The leakage current vs. bias voltage was measured before irradiaiton, after
1 krad, 7 krad, 35 krad, and 160 krad.

Total current as a function of time during Co exposure; average of two Hamamatsu sensors, two SEIKO sensors.

The total leak current at fixed bias voltages (Vb=120V for Hamamatsu,
75V for SEIKO) was traced after irradiation. The sensors were kept at 20 degC.
The bias voltage was off for the first 30 hr, then was kept on after except
the period when I-V characteristics were measured.

Total current as a function of time after Co exposure; average of two Hamamatsu sensors, two SEIKO sensors.

I-V curves at different doses

Changes in the interstrip capacitances

The interstrip capacitance C2 was measured 1 week after irradiation.
We notice an increase in the voltage where the interstrip capacitance is
minimized. Charge accumulation degrades the isolation between strips, thus
we require to apply larger voltage to do so.

Changes in the bulk capacitances

Testbench system

Production Sensors

A-class sensors are delivered to FNAL while some B- and C-class sensors
(sensor with *) are probed at Tsukuba in order to monitor the production
batches. The sensor number is laser printed on p-side in a binary.

Co-60 irradiation of Class B sensors

Three sensors H0196-198 were irradiated with Co-60 gammas to 0.2 Mrad.
Individual strip currents were compared before and after the irradiation.
The post irradiation measurements were made 2-4 days after (The sensors
are still underway of annelaing). The average leakage current of good
strips increased from 1 nA to 50 nA. Leaky strips (>100 nA or so) remain
to be leaky but there is no radiation induced increase in the leakage current
nor the neighboring strips are affected.

Long-term stability

In total seven sensors are kept biased at 120V in a thermostat chamber where
the temperature is controlled at 20degC. The total leakage current of these
sensors are measured at 120 and 150V. The bias was raised to 150V for a period
of ca.1min for this measurement.
Th seven sensors are of grade A (199,200,201), of grade B (195,202) and of
grade C (203,204) in terms of the IV characteristics (they fail dead channel fraction, though). See the IV curves measured by HPK (at 25degC).

We note the leakage current of 203 at 120V droped substantially in a couple of
days. To see what happened to this and to other sensors, the IV
characteristics was measured 5 days after biasing .
Comments: sensors 195 and 199 have been probed at Tsukuba. No instabilities are
seen for these sensors, instabilities being expected if the sensors are damaged.

Ladder construction procedure

The sensors are assembled into half-ladders, each consisting of 3 sensors.
Two half ladders are glued together, making a full ladder. The assembling is
performed at Fermilab with using assembling jigs constructed by Karlsruhe group.

(1) Half ladder frame.
Three sensors and a hybrid are glued on to this ladder frame.
The frame body is made of carbon fiber bars (black in photo) with Rohacel
(colored blue due to glue) foam. Since carbon is electrically conductive,
sensors are glued on Rohacel bars next to the long carbon fiber bars.
This part of Rohacel bar is reinforced by having a structure of stacking
Rohacel bars and carbon fiber bars.

(2) Sensor alignment stages.
Three stages for sensors and one smaller stage for hybrid with
teflon surface and vacuum chucking systems.
The stages can be individually positioned in X-Y (horizontal plane) and
rotated. The jig has two sets of linear bearings at the ends (one is out
of photo), using which a plate with the half-ladder frame placed can be
positioned vertically (see next photo).

(3) Hybrid is placed.
A plate is placed using linear bearings. On this plate a half-ladder
frame is positioned (temporarly) with spring-loaded pins at the four
sides. The teflon stage face is higher than the frame face so that
sensors and hybrid can be vacuum chucked without touching the frame.
The hybrid is positioned about a dowel pin, with the hybrid edge aligned
along the coordinate of the measurement system.

(4) Measure the dowel pin position.
Measure the position of the dowel hole made in the hybrid. The hole
center is calculated from 8 measurements around the hole. The coordinate
system is thus defined, and the fiducials will be positioned (see below)
according to the design. The half-ladder frame is re-positioned at this
stage.

(5) Place the 1st sensor.
The 1st sensor is placed. The r-phi strip side (n-side) is faced up.

(6) Place the 2nd sensor.
The 2nd sensor has been placed. Like for the 1st sensor, the two fiducials
along the median line are positioned. The TV monitor shows one of the
fiducial and cross-hair of the measurement system. Typically precision
of 2 microns is achievable. The 3rd sensor is similarly positioned.

(7) Apply the glue.
Epoxy glue (Hexcel 5313 with DEH24 hardener) is applied on Rohacel foam
using a syringe.

(8) Adjust the frame Z position.
The plate with the frame is lifted up by two balls underneath near
the linear bearings. The sensors and hybrid are to be faced to the frame.
At this stage, the sensor positions are re-adjusted to correct for
any shifts during the assembling process.
The whole process takes approximately two hours.
Wait for 24 hours till the glue is completely cured.
The vacuum is released and the position of each sensor is surveyed at
the four fiducials at the corners.

(8) Bonding.
Wire-bond to the fanout and between the sensors. The half-ladder frame
is designed such that the head of wirebonder is not interfered. Need
a long (vertical) head, though, in order to bond the backside (stereo
side). It takes typically 1.5-2 hours per side.
Two such half-ladders are mated on a stage (no photo) and epoxied
together using L-shaped carbon fiber pieces.

(9) Set on the spaceframe.
The ladders are placed on the spaceframe. The main parts are molded
carbon fiber rings with carbon fiber rods joining them together.
The photo is one side of the spaceframe. Identical another spaceframe
will be mated from right with rods joining them together.
Be plates glued on the spaceframe define the positions of the ladders.
Al cooling pipes run underneath the Be plates.

(10) Another photo of the spaceframe.
Two ladders are placed as a test, each for the inner and outer
layers of the forward barrel.

Production Sensors Delivery

The original plan and current status of the delivery as of December 23, 1999